Precision optical elements require highly polished surfaces of exacting figure and surface quality. The surfaces demand fabrication in proper geometric relation to each other and, where the elements are to be used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index.
Precision optical elements of glass are customarily produced via one of two complex, multi-step processes. In the first, a glass batch is melted in a conventional manner and the melt formed into a glass body having a controlled and homogeneous refractive index. Thereafter, the body may be reformed utilizing well-known repressing techniques to yield a shape approximating the desired final article. The surface figure and finish of the body at this stage of production, however, are not adequate for image forming optics. The rough article is fine annealed to develop the proper refractive index and the surface figure improved via conventional grinding practices. In the second method the glass melt is formed into a bulk body which is immediately fine annealed and substantially cut and ground to articles of a desired configuration.
Both processes are subject to similar limitations. The surface profiles that are produced through grinding are normally restricted to conic sections, such as flats, spheres, and parabolas. Other shapes and, in particular, general aspheric surfaces are difficult to grind. In both processes, the ground optical surfaces are polished employing conventional, but complicated, polishing techniques which strive to improve surface finish without compromising the surface figure. In the case of aspheric surfaces, this polishing demands highly skilled and expensive hand working. A final finishing operation, viz., edging, is commonly required. Edging insures that the optical and mechanical axes of a spherical lens coincide. Edging, however, does not improve the relationship of misaligned aspheric surfaces, which factor accounts in part for the difficulty experienced in grinding such lenses.
The direct molding of lenses to the finished state could, in principle, eliminate the grinding, polishing, and edging operations, which are especially difficult and time consuming for aspheric lenses. Indeed, molding processes are utilized for fabricating plastic lenses. Nevertheless, existing plastics suitable for optical applications are available in a limited refractive index and dispersion range only. Furthermore, many plastics scratch easily and are prone to the development of yellowing, haze, and birefringence. The use of abrasion-resistant and anti-reflective coatings has not fully solved those failings. Moreover, plastic optical elements are subject to distortion from mechanical forces, humidity, and heat. Both the volume and refractive index of plastics vary substantially with changes in temperature, thereby limiting the temperature interval over which they are useful.
The overall properties of glass render it generally superior to plastic as an optical material. Conventional hot pressing of glass, however, does not provide the exacting surface figures and surface qualities demanded for image forming optics. The presence of chill wrinkles in the surface and surface figure deviations constitute chronic afflictions. As observed above, similar problems can be encountered in conventional repressing techniques.
Various schemes have been devised to correct those problems, such devices frequently involving isothermal pressing, i.e., utilizing heated molds so that the temperature of the glass being molded will be essentially the same as that of the molds, the use of gaseous environments inert to the glass and mold materials during the pressing operation, and/or the use of materials of specifically defined compositions in the construction of the molds.
U.S. Pat. No. 4,481,023-Marechal and Maschmeyer shows and describes an improved mold for precisely pressing a glass preform which has an overall geometry closely similar to the desired final lens. A top and a bottom mold have molding cavities which precisely match the configuration of the final lens. A glass preform is heated to the molding temperature and the mold parts are separately heated. The molds are brought together against a ring having a thickness which governs the thickness of the lens to be molded.
In such a molding operation, the molding of the two opposed optical surfaces should be balanced. Balanced molding of a lens means that the degree to which the glass fills the voids between each mold surface is equivalent. This is typically measured by the radii between each lens surface and the common side wall.
It is an object of the present invention to achieve balanced molding of lenses by using adjustments to the closing movement of the molds.